1. Field of the Invention
The present invention relates to an image reading device adapted for use in an image information processing apparatus such as a facsimile, an image scanner, a copying machine or the like, and more particularly to an image reading device and a photoelectric converting element capable of converting optical signal not only of visible spectral region but also of invisible spectral region into electrical signal.
2. Related Background Art
In the field of image reading device, there are already known a charge-coupled device (CCD), a MOS device and an amplifying device formed by connecting a capacitative load to the emitter of a phototransistor, as disclosed in the U.S. Pat. No. 4,791,469 granted to the inventors T. Ohmi and N. Tanaka.
Also there are recently conceived various applications, and image reading devices having novel functions are being required.
For example the copying machine is required, in addition to the improvement in image quality and the color image reproduction, to have the ability to recognize, reproduce and record an invisible image to the human eyes.
An example of such invisible image is the image formed with ink absorbing infrared light.
In general, the sensor for detecting the invisible light is an individual device, and requires a new design concept for effecting image detection, in combination with a sensor for detecting the visible light.
As a basic design concept, the present inventors already found a technology of incorporating a sensor for visible light detection and a sensor for invisible light detection in a single semiconductor chip. However, said technology still has a room for improvement.
On the other hand, the image quality improvement and the color copying capability of the copying machine has brought the danger of forgery of banknotes, stamps, valuable securities etc. into reality. For this reason, in the recognition of banknote or the like, there have been devised various methods such as detection of the stamp pattern on the banknote.
Also utilizing the fact that the pattern of the original image is formed with certain color hues, the present applicant already proposed a method of recognizing the banknote etc. from the color hue of said original pattern.
Also certain banknotes enable recognition of the true one and the false one, by printing a predetermined mark with fluorescent ink which emits visible light when irradiated with ultraviolet light.
Also for forming such predetermined mark, the use of ink capable of absorbing infrared light was also proposed by the present applicant.
In such infrared light detecting device, the Japanese Patent Application Laid-open No. 4-286350 of which U.S. counterpart is U.S. Ser. No. 139174 entitled "Image Processing Apparatus and Method Therefore" filed on Oct. 21, 1993 proposing to achieve making compact of the device and easy optical adjustment by a monolithic structure of a sensor for ordinary color image formation and a sensor for infrared light detection, thereby enabling to use a common optical system.
However, such conventional system is difficult to design in common for plural valuable securities, because a visible pattern is the target of recognition. Therefore, for distinguishing the valuable securities of N kinds, it has been necessary to select the features of N kinds in advance and to independently recognize each security, and it has been difficult to realize such apparatus inexpensively.
Also a CCD sensor for reading the images of visible and infrared regions by separating the spectral regions additionally requires, in comparison with the conventional sensor, an optical filter for reading the infrared light and an increased number of elements or lines of the sensor, whereby the sensor itself and the post-processing system therefor become complex and a decrease in the light-receiving area of the sensor or an increase in the size of the sensor is unavoidable.
Also, since the sensor elements for the visible light and those for the infrared light are arranged on a same plane, at least one of the sensors may become out of focus, due to the difference in the focus position.
Also in case of using such sensors for respectively reading the infrared information and the visible information, it becomes necessary to clearly separate the visible information and other information.
Furthermore, for obtaining a satisfactory resolving power on a monolithic CCD sensor over a wide spectral range from visible to near-infrared region or from visible to near-ultraviolet region, there is required a significantly increased number of lenses, leading to an increased cost and a larger space of the device. Also in an optical system employing a short-focus lens array, it has been impossible to maintain a constant resolving power over a wide spectral range, because such lens array is composed of single lenses.
Also the conventional image reading device employed in the office equipment such as copying machine is composed for example of a CCD or a MOS sensor requiring a long optical path, or a contact image sensor employing amorphous silicon, and such image reading device is sometimes combined with color filters for the color image reproduction.
However, such photoelectric converting device combined with filters is not necessarily superior, in terms of spectral sensitivity and resolving power for infrared light detection, to the device for visible light detection, and still has a room for improvement.